Abstract Molecular beam results indicate that the dissociative chemisorption of neopentane on Pt(111) occurs by both direct collisionally activated and trapping-mediated mechanisms. Direct dissociation dominates at translational energies greater than about 110 kJ mol −1, exhibits normal energy scaling and is nearly independent of the surface temperature. The trapping-mediated pathway, which displays a significant temperature dependence, is the controlling dissociative adsorption mechanism at gas-molecule translational energies less than 110 kJ mol −1. Experiments performed on Pt(111) surfaces with differing defect-site densities indicate that trapping-mediated dissociation is facilitated by surface defects. The measured surface-temperature dependence of the trapping-mediated dissociation probability was accurately reproduced by a model which assumes that the rate of migration to defects is rapid compared to the dissociation and desorption rates.